1. Field of the Invention
This invention relates to a process for preparing an electrophotographic photosensitive member, and more particularly to a process for producing a photoconductive layer from a mixture system of photoconductive particle and a binder resin.
2. Description of the Prior Art
In general, a photosensitive member for electrophotography comprises a base and a photoconductive layer overlying the base, or if desired, additionally an insulating layer overlying the photoconductive layer. The structure of the photosensitive member is dependent upon the electrophotographic system.
A representative electrophotographic process using a photosensitive member having a photoconductive layer overyling a base is so-called Electrofax system which comprises applying a positive or negative voltage to a surface of the photoconductive layer, subjecting to imagewise exposure by radiation energy to form electrostatic latent images on the surface of the photoconductive layer, and developing the latent image in the dark by a dry or wet development to reproduce the images.
Another representative electrophotographic process using a photosensitive member having an insulating layer on a photoconductive layer is a process as disclosed in U.S. Pat. No. 3,666,363 and U.S. Pat. No. 3,438,706 which comprises applying a primary voltage of a certain polarity to an insulating layer of the photosensitive member, applying or discharging a secondary voltage of a polarity opposite to that of the primary charge simultaneously with imagewise exposure by radiation energy, if desired, additionally applying a blanket radiation to produce electrostatic latent images on the surface of the insulating layer, and developing the latent images by dry or wet development to reproduce the image.
Other than the above mentioned electrophotographic processes, there are proposed various electrophotographic processes which fundamental principle using the photoconductive layer is almost the same as that of the above mentioned electrophotographic processes.
The photoconductive layer may be composed of a photoconductor alone or a mixture of photoconductive particle and a binder resin. Examples of the former are alloys containing Se, Se-Te or Se-Ge-Si, vapor deposited CdS and organic semiconductors. The latter may be produced by dispersing crystal particles of Cu.sub.2 O, CuI, ZnO, ZnS, ZnSe, CdS, Se-Te, CdSe, CdTe, PbS, Sb.sub.2 S.sub.2, GeS, or GeSe in a binder resin such as thermosetting resins, for example, epoxy resin, and unsaturated polyester resin, and thermoplastic resins soluble in a solvent, for example, copolymer of vinyl chloride and vinyl acetate, polyvinyl chloride, polyvinyl acetate, cellulose acetate, nitrocellulose, acryl resin, methacryl resin, phenolic resin, polyvinyl alcohol, and polyvinyl butyral and forming a photosensitive layer therefrom.
The present invention is relevant to the latter, that is, photoconductor particle dispersed in a binder resin.
In general, the amount of the binder resin in such photoconductive layer is preferably not higher than 50% based on the photoconductor particle since the amount of the binder resin exceeding 50% results in high electric resistance, lowering the characteristics, and difficult formation of the image. Therefore, heretofore there has been prepared a paint containing far more photoconductor particle than a binder resin, diluted with a solvent to a viscosity suitable for coating, applied to a support and dried (cured).
Photoconductive layer of an electrophotographic photosensitive member should have good charge retaining property, low dark decay, high conductivity when irradiated and high sensitivity. Further, a binder resin should have the following characteristics. (1) When photoconductor particle is dispersed in the binder resin, the resulting surface is free from irregularity and smooth. (2) The resulting photoconductive layer is free from void and has uniform density. (3) The binder resin can highly disperse the photoconductor particle and shows high adsorption property for the photoconductor particle.
The above-mentioned characteristics affect image quality such as electrostatic contrast, resolving power and the like to a great extent.
According to conventional processes for producing a photoconductive layer as mentioned above, there are selected a binder resin, a diluent and photoconductor particle taking into consideration chemical composition, molecular weight, and viscosity. Furthermore, the coating method and the manufacturing temperature have been improved based on experience to satisfy the above mentioned characteristics. However, the conditions can not be determined on the basis of the same rule. In other words, even when the best dispersion and uniformity can be obtained by selecting particular chemical composition and molecular weight, the resulting photoconductive layer is not always excellent as a photosensitive member for electrophotography. On the contrary, even when the charge retaining property and sensitivity are excellent, sometimes there occurs irregularity of charge and lowering of conductivity upon irradiated. Therefore, it is very difficult to find definite conditions capable of always producing a good photoconductive layer. It is very significant to find such a condition capable of giving always a good photoconductive layer.
The present inventors have now found that there is a certain important condition for obtaining such good photoconductive layer which is closely related to adsorption property between a binder resin and a photoconductor particle. The present invention has been accomplished based on this discovery.